Ralph Gebauer

Interfacial engineering is an important aspect for the development of high-efficiency photoelectric devices. The microscopic interplay between lattice distortion and carrier transport remains poorly understood. This study utilizes constrained density functional theory (CDFT+$U$) and Marcus theory to bridge this gap, using the anatase (101)/(001) interface as an example to investigate charge separation with a particular focus on the role of polarons. We find that while the interface undergoes distortion of the interfacial structure, both electron and hole polarons strongly localize with trapping energies of \ensuremath{-}0.48 eV and \ensuremath{-}0.97 eV, respectively. Quantitative analysis via crystal field theory reveals that the polaron stability is governed by the magnitude of orbital splitting (up to 4.47 eV for electrons and 3.86 eV for holes), which dictates a spatial disparity where both carriers initially favor the (001) side. However, dynamic transport simulations uncover an asymmetric kinetic steering mechanism that overrides these static energy preferences: Electron migration: Electrons exhibit a hierarchical transport pathway, initial intraplane diffusion followed by interface-guided migration, resulting in mobility toward the (101) side that is higher than toward the (001) bulk. Hole migration: Holes generated on the (101) side follow a cascaded mechanism, preferentially penetrating the interface to settle on the (001) side. By demonstrating that efficient charge separation is a product of electronic coupling and reorganization energy rather than mere spatial distribution, this work provides a predictive framework for optimizing photocatalytic efficiency through polaron-aware interface design.

Ferroelectric tunnel junctions (FTJs) have great potential for high-density nonvolatile memory applications. Achieving high performance of FTJs in low-dimensional materials is still challenging. Here, we systematically investigate the structural and electronic properties of dual-polarized GaAs/In2Se3 heterostructures by first-principles calculations based on density functional theory. It is shown how the interplay of polarization and stacking geometries of the monolayers can lead to heterojunctions with vastly different electronic properties. The band alignment in the heterojunctions is effectively governed by the ferroelectric polarization, changing from type-II (P↑ state) to type-III (P↓ state) in a self-doped P–N contact. Particularly, it is demonstrated that ferroelectric heterostructures can achieve a remarkably high tunneling electro-resistance ratio of 5.06 × 1014% at zero bias and up to 108% under a bias of ±0.15 V. This characteristic behavior is strongly related to the interfacial charge redistribution and to the ensuing electric polarization. Our findings not only unravel the role of polarization on the electronic properties but also open perspectives for nonvolatile FTJs in 2D ferroelectric semiconductors.

Short hydrogen bonds, defined by donor-acceptor distances of less than 2.5 Å, represent a distinct regime in acid-base chemistry where conventional models of hydrogen bonding break down. In an organic crystal formed via a temperature-induced chemical transformation of l-glutamine, we previously identified a short hydrogen bond featuring a double-well potential indicative of an activated proton transfer. Here, using path-integral ab initio molecular dynamics, we show that nuclear quantum effects completely eliminate the classical barrier leading to a symmetrization of the proton along the hydrogen bond. Classically, proton transfer is strongly coupled to the rocking motion of a neighboring ammonium ion; under quantum effects, this coupling is significantly reduced. Furthermore, examination of the electronic structure through Wannier centers reveals a quantum-driven redistribution of bonding electrons, blurring the distinction between hydrogen bonding and covalency. Taken together, our findings indicate that nuclear quantum effects in this organic crystal create a regime in which the donor and acceptor simultaneously act as the acid and base.

Hydrogen, as the most abundant element in the universe, holds immense potential as the fuel of the future due to its high energy density per unit weight and its environmentally friendly nature. This article offers a comprehensive overview of recent theoretical advancements in hydrogen storage, outlining a general framework for achieving practical hydrogen uptake. We examine the fundamental interaction mechanisms, emphasizing orbital hybridization, polarization induced by external electric fields, and multipole Coulomb interactions between metal atoms and dihydrogen. Special focus is placed on calcium metal, which exhibits a transition from electrostatic to Kubas-type orbital interactions as multiple hydrogen molecules adsorb. The processes of hydrogen dissociation and spillover on sorbent surfaces, catalyzed by metals, are also discussed. We present the formalism by Lee . [Phys. Rev. Lett. 97, 056104 (2006)] as a method for calculating maximum hydrogen adsorption per site under varying temperatures and pressures, facilitating estimates of reversible hydrogen delivery. General computational strategies are reviewed, highlighting potential sources of error, such as neglecting zero-point vibrational energies, basis set superposition errors, and inaccuracies due to inappropriate density functionals. Additionally, we address practical challenges in designing optimal hydrogen storage materials for real-world applications, including nanostructure breakdown under intense electric fields, metal clustering, and oxygen blockage of metal functional sites. The article concludes by suggesting strategies to bridge the gap between computational simulations and experimental results, guiding the design of next-generation hydrogen storage materials.